The present invention relates to implantable electrodes, and more particularly to an implantable electrode configured for minimally invasive implantation within the basal end of the scala tympani duct of a human cochlea. Such electrode may be used with a hybrid cochlear stimulator of the type described in copending patent application, Ser. No. 09/569,696, filed May 12, 2000 (which application claims the benefit of U.S. provisional applications Ser. No. 60/134,289, filed May 14, 1999, or application Ser. No. 60/155,840, filed Sep. 24, 1999), which copending application is assigned to the same assignee as is the present application, and is incorporated herein by reference. Alternatively, such electrode may be used with any type of electrical stimulator to compensate for hearing loss in the high frequency range, and/or to provide electrical stimulation to suppress tinnitus.
Hearing loss is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
Sensorineural hearing loss, on the other hand, results due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. Persons who suffer from sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, there have been developed numerous cochlear implant systemsxe2x80x94or cochlear prosthesisxe2x80x94which seek to bypass the hair cells in the cochlea by presenting electrical stimuli directly to the ganglia of the auditory nerve located adjacent the modiolar wall of the cochlea. When triggered, the ganglia, also referred to as ganglion cells, send nerve impulses to the brain via the auditory nerve, leading to the perception of sound in the brain, and an at least partial restoration of hearing function. The common denominator in these cochlear prosthesis systems has been the implantation into the cochlea of electrodes which are responsive to a suitable external source of electrical stimuli and which are intended to transmit those stimuli to the ganglion cells, and thereby to the auditory nerve fibers.
It is estimated that a large segment of the hearing-impaired population exhibit sensorineural hearing loss relative to high frequency sounds, but maintain the ability to transduce middle-to-lower frequency sounds through functioning hair cells. For this segment of the population, there is thus a need for a xe2x80x9chybridxe2x80x9d cochlear stimulation system that electrically stimulates only the ganglion cells responsible for sensing higher frequency sounds, while allowing the normal hearing process to function for the purpose of sensing lower frequency sounds.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. Because the ganglion cells responsible for sensing higher frequency sounds are all generally located in or near the base end of the scala tympani duct of the cochlea (the end of the scala tympani duct nearest the round window), a hybrid cochlear stimulation system thus requires at least one electrode that can be inserted into the cochlea so as to be near such cells, but so as to not block nor significantly interfere with the normal functioning of the cochlea for hair cells located deeper within the cochlea. In order not to interfere with the normal functioning of the cochlea, such electrode must be implantable or insertable into the base end of the cochlea in a minimally invasive manner, i.e., so as no to significantly interfere with the normal functioning of the round window membrane, or so as not to cause a reaction in the form of tissue or new bone formation, all of which may interfere with and adversely affect residual hearing within the implanted ear. No such electrode, or electrode system, currently exists to applicants"" knowledge.
It is a feature of the present invention to provide an electrode system suitable for use with a hybrid cochlear stimulation system.
It is another feature of the invention to provide mechanical access into the cochlea without destroying or damaging the ability of the cochlea to perceive sound through the round window membrane in a natural manner. The mechanical access thus provided by the invention advantageously provides an opening through which a relatively short cochlear electrode array may be removably inserted, and/or through which drugs, steroids, or other medicinal or tissue-growth-inhibiting fluids may be readily delivered into the cochlea.
It is yet another feature of the invention to provide a cochlear electrode system that may be inserted into the cochlea in a minimally-invasive manner, and which electrode system can thereafter function without interfering with or damaging the normal operation of the round window, thereby allowing mechanical movement of the round window to set up fluid waves within the cochlea, in conventional manner.
It is an additional feature of the invention to provide a cochlear electrode system that can be inserted into the cochlea of a patient while preserving the natural hearing ability of the patient.
It is still another feature of the invention to provide a cochlear electrode system that may be removably inserted into the cochlea without causing damage to the basilar membrane.
The present invention addresses the above and other needs, and satisfies the above features, by providing a grommet that can be readily screwed into an opening made in the bony tissue adjacent the round window. Threads included on the outside of the grommet engage the bony tissue and form a tight seal that holds the grommet securely in position. Such tight seal also prevents cochlear fluids from escaping from inside of the cochlea.
The grommet further includes an access hole in its center. Such access hole provides an opening, i.e., mechanical access, into the base end of the cochlea. Fluids, e.g., drugs, steroids, tissue-growth-inhibiting fluids, and the like, may be controllably delivered through this access hole into the cochlea. Also, through this access hole, a relatively short electrode array may be inserted. Such electrode array has from four to eight, e.g., five, electrode contacts spaced apart along a flexible carrier. The flexible carrier is about 6-8 mm in length. In conventional manner, each of the electrode contacts is connected to a respective wire embedded within the carrier. The wires exit a proximal end of the carrier via a wire bundle. The wire bundle, in turn, is connectable to an implantable cochlear stimulator (ICS), or equivalent pulse generator.
Advantageously, the access hole through the grommet, while preferably round, may also be oval or oblong in shape, or otherwise keyed, to match a cross section of the carrier of the electrode array. Thus, with such shape or keying, when the electrode array is inserted through the access hole, it must assume a prescribed orientation, i.e., an orientation that positions the electrode contacts so that they must face the modiolar wall of the cochlea. Further, such prescribed orientation prevents the electrode array from twisting or turning.
The carrier of the electrode array is made from a suitable pliable substance, such as silicone rubber, or Silastic. Such pliable substance, in combination with a taper in the carrier body that provides a larger carrier cross section near a proximal tip of the electrode array than exists at a distal end of the electrode array, allows the electrode array to be tightly inserted into the access hole when fully inserted therein so as to completely close off and seal the access hole. With such a tight seal, the cochlea thus remains filled with fluid which can activate hair cells as fluid waves are established through motion of the round window membrane, which is the normal process for hearing.
In use, electrical stimulation is provided only to the base end of the cochlea through the electrode array to supplement hearing of high frequency sounds. Normal hearing processes (activation of hair cells through fluid motion) occur at the apex and middle regions of the cochlea for sensing lower frequency sounds. As needed, such normal hearing processes may be supplemented by conventional or custom hearing aid apparatus, including in-the-ear-canal or middle-ear hearing aid devices.
It is thus an object of the present invention to provide an electrode array that may be inserted into the base end of the cochlea of a hearing-impaired patient so that ganglion cells located near the base end of the cochlea may be stimulated directly with electrical stimuli, thereby enhancing the ability of the patient to sense high frequency sounds.
It is a further object of the invention to provide an electrode array suitable for insertion into the base end of the scala tympani duct of a human cochlea without destroying the function of the basal membrane (the basal membrane separates the scala tympani from the scala vestibule, another one of the ducts within the cochlea), and without causing fluid to escape from the scala tympani, or other ducts of the cochlea, thereby allowing normal hearing processes (fluid motion activation of hair cells) to continue to occur in the cochlea simultaneously with electrical stimulation.